They are critical to understanding the concepts of the content. Generally, they have low-frequency use and are limited to specific knowledge domains. Examples would include words such as isotope, peninsula, refinery. These words are best-learned when teaching specific content lessons, and tend to be more common in informational text.
tl;dr:
Using clear and specific language in reference to a subject ensures understanding and correctness. For example, always referring to a proton as a proton, and not just a subatomic particle, in a chemistry essay makes sure no one gets confused and thinks you could be referring to electrons or neutrons. Saying "grocery store" instead of just "the store" when going shopping for food, or "boiling" instead of just "cooking."
